The present invention relates generally to the field of suction lines, and more particularly, relates to suction lines employed in suppression pools of a boiling water reactor (BWR) nuclear power plant.
Boiling water reactors are operated within a containment or reactor building. The containment structure is a physical boundary against release of fission products to the environment following an inadvertent release of those products from a reactor coolant system. The containment structure with its associated systems is designed to withstand a set of postulated internal and external loads and to effectively contain radioactivity that may be released from the primary system. It also provides a shielding function, namely the protection of personnel from exposure to radioactivity contained inside the containment.
Nearly all operating BWR plants have a pressure suppression primary containment consisting of a xe2x80x9cdry-wellxe2x80x9d containing the reactor vessel and primary systems components, and a suppression pool or xe2x80x9cwet-wellxe2x80x9d containing a large volume of water to condense steam discharged from the primary systems. The wet-well also serves as a short term heat sink. A variety of pressure suppression primary containment designs are used in BWR plants; however, substantially all are functionally similar and all are within a secondary containment.
The suppression pool water (i.e. wet-well water) also provides a source of water for one or more emergency core cooling systems (ECCS). An ECCS is designed to inject water back into the reactor vessel to make up for lost water in the event of a loss of coolant accident (LOCA). The ECCS also re-circulates water through the core following a LOCA to provide for long term post-accident core cooling. To accomplish these purposes, the ECCS includes a plurality of suction lines that penetrate and draw water from the wet-well. In order to protect the integrity of the ECCS components, and to prevent blockage of the suction lines, the suction lines are provided with strainers to filter out harmful debris.
It has been recognized that conventional suction strainers installed on ECCS suction lines currently installed in BWR plants might not be able to handle the amount of debris expected to result from a LOCA. As a result, conventional strainers are being replaced with trains of strainers that are much larger and heavier. The replacement trains of strainers connect to a suction pipe system through a penetration in the wet-well wall. Because of their larger size, the trains include more inertial mass that cause the strainers, and the attached suction pipes, to move significantly during seismic and other events. For example, when a large pressurized pipe ruptures with great force, resulting in an LOCA, the suction strainers in the wet-well are subjected to large hydrodynamic forces that can deflect the suction strainers and subject the attached suction pipes to large reactive forces that may occur in any direction. Conventional penetration designs that link the water in the suppression pool to re-circulation piping are unable to handle the resultant inertial forces acting upon the large trains of strainers.
Penetrations in the wet-well are themselves complex connections that must be properly designed and installed to avoid leakage. Such penetrations must accept any applied load in any direction, and also must prevent leaks. Existing penetrations are not likely to be able to support new loads caused by the greater weight of the new trains of strainers attached to each penetration.
It is therefore an overall object of the invention to provide a flexible penetration attachment for attaching large trains of strainers to a boiling water reactor wet-well suction pipe.
It is a further object of the invention to provide a flexible penetration attachment that is structurally able to accept any loads in any direction acting upon large trains of strainers.
It is still another object of the invention to provide a flexible penetration attachment that avoids leakage while accepting any loads in any direction acting upon large trains of strainers.
These and other objects of the present invention are realized through a flexible penetration attachment for strainers that allows a suction pipe to communicate with liquid in a wet-well through an aperture in the wet-well wall. The penetration includes a boot, preferably fabricated as a unit, for defining an outer circumference of the aperture. Suction pipe extends through the boot such that an annular space is defined between the aperture outer circumference and an outer surface of the suction pipe. A resilient seal having a generally U-shaped cross section with first and second ends is placed within the annular space. The first end is fixedly secured to the outer surface of the suction pipe and the second end is fixedly secured to the aperture outer circumference. The resilient nature of the seal allows the suction pipe to move a pre-determined distance in response to a force in a radial or transverse direction. Securing the resilient seal between the outer surface of the suction pipe and the aperture outer circumference also allows the suction pipe to move a predetermined amount in response to a rotational force. The resilient seal thus prevents leakage from the suppression pool while allowing the suction pipe to move in response to all forces placed upon it.
A flexible penetration attachment as described herein may therefore be used to connect replacement trains of strainers in a wet-well pool with ECCS suction lines through existing wet-well penetrations. The resilient seal flexes in response to transverse, lateral and rotational applied loads. As a result, the flexible penetration attachment minimizes leakage through the penetration while absorbing loads applied to the combined trains of strainers and the suction line.